Is the mother sinful for gifting one of her apartments to her daughter without the rest of her male siblings, and how can the inheritance be divided and the matter rectified after the death of one of the children?

If a mother gifts an apartment to each of her children, then she has acted justly. However, if she singles out one of them for the gift of an apartment without a legitimate reason, she must either revoke the gift or give an equivalent gift to the rest of her children. If she dies before doing so, and the gift is in the possession of the donee, the majority of scholars hold that it is valid. It is, however, recommended for the recipient of such an unfair gift to return it to the estate so that it can be divided justly.

As for the division of inheritance: this occurs after the unfair gift is returned to the estate. Then, all heirs are identified, and the wealth is distributed among them. The share of the deceased brother goes to his legal heirs. It is advisable to consult a Sharia court or scholars for the division of wealth.